Recent years, high performance is remarkable in electronics represented by portable telephones and communication terminals. And, for these electronics is used a printed wiring board having IC chips mounted thereon. As a method of mounting IC chips on a printed wiring board, a flip chip type of directly mounting IC chips on the surface of a printed wiring board is widely employed. As such a printed wiring board, there has been known the one comprising a core substrate, a build-up layer formed on the core substrate and connecting pads for mounting an IC chip through a solder bump on the surface of the build-up layer.
Here, as a printed wiring board, use are made of epoxy resin, BT (bismaleimide triazine) resin phenol resin and the like formed with reinforcement such as glass fiber or the like. The thermal expansion coefficient of these core substrates is about 12-20 ppm/° C. (30-200° C.) and about 4 times larger than that (about 3.5 ppm/° C.) of silicon of the IC chip package.
Therefore, the flip-chip type has, in case of repeating temperature change with heat generation, the possibility of destroying the solder bump due to differences in thermal expansion and heat contraction between the IC chip and the core substrate. For solving such problem, there is proposed a method of interposing an interposer having a thermal expansion coefficient between the printed wiring board and the IC (refer to Japanese unexamined patent application No. 59-996).